I finished all the DLC and everything but I'm only level 29 still... Rrrrr.
One thing I JUST realized yesterday is that if you play with companions (like the dog, or charon or something), when they kill the enemy, you get 0 experience - unless you deal 50% of the damage to the enemy or something. I found the companions to be REALLY powerful, since they apparently can't seem to die, and keep dealing good damage. I played with them for a little while, when I realized they were not only robbing me of challenge (I play on Hard Mode and enjoy the difficulty), but also XP now!
Also, UP your difficulty if you wanna raise levels, 150% normal XP if you play in VERY HARD mode. You're only 1 away from the level cap though.